In oil and gas exploration and exploitation, marine seismic surveys are an important tool for making drilling-related decisions. Seismic data acquired during such a survey is processed to generate a profile, which is a three-dimensional approximation of the geophysical structure under the seafloor. This profile enables those trained in the field to evaluate the presence or absence of oil and/or gas reservoirs, which leads to better management of reservoir exploitation. Enhancing seismic data acquisition and processing is an ongoing process.
FIG. 1 is a vertical-plane view of a generic marine survey setup 100. A vessel 101 tows a seismic source 102 (note that, for simplicity, the source's full configuration is not shown) and streamers (only one streamer 104 is visible in this view) in a towing direction T. When the seismic source is activated, seismic energy is emitted into the water and propagates into the rock formation under the seafloor 110. The seismic energy is partially reflected and partially transmitted at interfaces where the acoustic impedance changes, such as at the seafloor 110 and at an interface 112 inside the rock formation. Reflected energy may be detected by sensors or receivers 106 (e.g., hydrophones, geophones and/or accelerometers) carried by the streamers. The seismic data represents the detected energy.
The bird's-eye view in FIG. 2 shows that, upon activating seismic source 200, seismic data corresponding to an illumination area 210 having a width I1=D is acquired with a streamer spread 220 having a width L1=2×D. The illumination width is determined by multiple factors, such as the magnitude of emitted energy, energy attenuation along the propagation path from the source to the receiver via the underground formation, noise level, depth range of interest, receiver's sensitivity, etc. In this document, the term “illumination width” refers to a cross-line width that can be explored using a reference source.
Illumination area 210 is a locus of mid-points between the source activation location and receivers detecting reflected energy. If a receiver is at a cross-line distance x from the source activation location, then the mid-point is at a cross-line position x/2 from the source activation location. The term “cross-line” indicates a direction perpendicular to the towing direction T and to gravity g (both shown in FIG. 1). The streamer spread is made of plural streamers towed underwater in such a way as to maintain predetermined cross-line distances from one another. Six streamers towed at 50 m cross-line distance between adjacent streamers form a streamer spread with a cross-line width of 250 m. The streamer spread cross-line distance is the distance between the leftmost and rightmost streamers. For a spread configuration having M streamers with a distance between two consecutive streamers of d, the width is (M−1)×d.
In order to acquire high-resolution data with this conventional marine seismic data acquisition system, the distance between adjacent sail lines (which are suggested by the arrows pointing in the towing direction) is S1=D. Here the sail lines are defined by the streamer spread's trajectory, more specifically by the trajectory of the middle of the streamer spread. The bottom half of FIG. 2 illustrates the data acquisition system sailing along a sail line adjacent to the sail line along which the data acquisition represented in the upper half sails. High sail line density takes a long time and is, therefore, expensive.
Thus, there is a need to provide data acquisition systems and methods that would acquire high-resolution seismic data faster and at less cost than conventional approaches.